An electric power generator using a free piston engine is known, for example, in a PCT JP-Publication No. 2003-519328 (which corresponds to U.S. Pat. Nos. 6,397,793 and 6,276,313).
The electric power generator disclosed in this publication has the free piston engine and a power generation means. The free piston engine includes two opposed pistons in a cylinder and back pressure chambers as air spring means respectively arranged at back sides of the pistons, and the power generation means has an electromagnet for transforming kinetic energy of the pistons into electric energy.
In the free piston engine, mixed gas in a combustion chamber formed between the pistons is auto-ignited by being compressed when the pistons move closer to each other. An explosion of the ignited gas generates a driving force to push the pistons in directions in which the pistons move away from each other. At this time, back pressure chambers are compressed and then the pistons are pushed back in the opposite directions, that is, directions in which the pistons move closer to each other. Repetition of these movements causes back-and-forth movements of the pistons, and the power generation means produces electric power by transforming the kinetic energy of the back-and-forth movements of the pistons to the electric energy.
In the electric power generator, the power generation means applies a force to each of the pistons so that the pistons move synchronously, that is, the heading directions of the pistons are always opposite and a phase difference of their back-and-forth movements is 180 degrees.
In the free piston engine, a combustion condition (e.g. a combustion timing) changes depending on a temperature, an air-fuel ratio, and a density distribution of the mixed gas, because the mixed gas is auto-ignited as a result of the compression of the mixed gas, unlike an engine in which the mixed gas is ignited by a spark plug.
Therefore, depending on the temperature, the air fuel ratio of the mixed gas and so on, the mixed gas may be auto-ignited in advance of an optimum timing during a compression stroke (i.e. a timing for most efficiently transforming the energy of fuel to the driving force of the pistons), even if the synchronization of the pistons is achieved. In other cases, the mixed gas may not be auto-ignited even when the compression stroke has ended and the pistons start getting away from each other (that is, misfires of the free piston engine). Therefore, it is difficult to operate the conventional piston engine with constant efficiency. In other words, it is difficult to make the pistons efficiently move back-and-forth. The inefficient operation of the free piston engine would result in the inefficient generation of the electric power using the free piston engine.